1. Field of the Invention
The present invention generally relates to excimer laser ablation of materials, and more specifically to a method and apparatus which uses excimer laser ablation to form devices and delineate patterns for electronic microcircuit fabrication and the like.
2. Description of the Related Art
Device structures and patterns such as mesas and microlenses are conventionally formed at the surfaces of semiconductor materials such as cadmium telluride (CdTe) by photolithography in combination with wet chemical etching. A typical solution used to perform such etching is bromine-/ethylene glycol. A number of alternative etching solutions are discussed in a paper entitled "Etching of Cadmium Telluride", by P. Gaugash et al, J. Electrochem. Soc., SOLID-STATE SCIENCE AND TECHNOLOGY, vol. 128, no. 4, (April 1981), pp. 924-926.
The disadvantages of wet chemical etching for semiconductor processing include the generation of large quantities of toxic wastes. Other problems include surface contamination by solvent impurities, and the lack of process control leading to, for example, undercutting of etched surfaces, unwanted changes in surface composition, and crystallographic dependence on etching rates.
Generally, the use of photolithography and wet chemical processing for microcircuit device delineation causes semiconductor material surfaces to be exposed to many chemical processes, each with the potential to modify the electrical properties and/or chemical composition of the surface.
The use of dye laser irradiation for surface preparation of CdTe in an ultraviolet environment is presented in an article entitled "CHARACTERISTICS OF CADMIUM TELLURIDE SURFACES PREPARED BY PULSED LASER IRRADIATION", by V. Montgomery et al, Thin Solid Films, 124 (1985), pp. 11-17. Montgomery reported that CdTe surfaces subjected to treatment by bromine-methanol, which is used extensively as a polishing etch for this material, were highly contaminated with carbon and oxygen. Irradiation with a laser beam at relatively high fluences in excess of 100 mJ/cm.sup.2 resulted in reducing the oxygen content to a negligible level. However, the carbon content remained excessive, even at fluences as high as 400 mJ/cm.sup.2. Also, irradiation by the dye laser produced tellurium rich surfaces except under the lowest fluence conditions.
Montgomery teaches how to minimize these detrimental effects by replacing the conventional rinse with an alternative rinse procedure which was designed to reduce carbon on GaAs surfaces. Although the surface contamination was effectively removed with relatively low disruption of the underlying CdTe surface, fluence levels of 200 to 250 mJ/cm.sup.2 were required, which effectively limited the beam diameter to approximately 3 mm.
Excimer lasers which operate at ultraviolet wavelengths (e.g. 308, 248, 193, 157 nm) have been advantageously employed for etching of polymer and biological materials. A typical example is found in a paper entitled "Excimer laser etching of polymide", by J. Brannon, J. Appl. Phys., 58(5), (1 Sept. 1985), pp. 2036-2043. Etching of HgCdTe using an excimer laser to convert planar waveguides into strip waveguides is reported in an article entitled "Assisted ArF excimer photo-etching of Mercury Cadmium Telluride (MCT) semiconductor", by A. Azema et al, SPIE vol 998, Excimer Beam Applications (1988), pp. 72-75. Although etching of the MCT material was accomplished, the central part of the illuminated zone exhibited extreme damage, including the formation of mercury nodules which render the material surface area unusable for fabrication of operative electronic devices therein.